Remember the M body with the 2.4 rear gears, it had that crappy ratio working against it in all situations but steady state highway speeds, in which it squeaked out a bit more mpg, still got hit with the gas guzzler tax though. Whereas if it would have had an OD trans and 3.55 gears it would have done better around town and still could have got the same mpg on the highway, Don't hamper yourself with too tall a rear gear ratio.